I will throw in my $0.02 a caravat I am new to this game as well. I have a new to me sd 4 which I believe is the same as the sd4 but a real case and a couple minor improvements.
When I was doing my research a friend that does 3-d printing warned me these things can be fiddly. that goes for any hobby level 3-d printer.
you need to calibrate the extruder level the bed set the Z home switch so the first layer prints on the bed not above it and learn the software and settings.
As far as stability I have only had my machine a week . But as with any device things can loosen and need to be adjusted. belts will stretch over time etc.
A good working knowledge of the 3-Cartesian plane is a must.
also unless you plan to uses canned parts you need a knowledge of parametric design software.
IMHO cubify invent is a good value for $50 and you can try before you buy for 14 days.
I know there are a couple people on this forum that are frustrated owners right now. so do your homework and check the pros and con on the various brands.
Tin
Soliddoodle 4 stock w glass bed------Folger Tech Prusa 2020 upgraded to and titan /aero extruder mirror bed
FT5 with titan/ E3D Aero------MP mini select w glass bed
MP Utimate maker pro-W bondtech extruder
Marlin/Repetier Host/ Slic3r and Cura